Today, more than ever, network operators are looking for solutions that will cover the most modern telecommunications trends, in term of efficiencies and traffic capability. In wireless networks an efficient solution in terms of capacity per Hertz of bandwidth used is to transmit two RF signals with Co-Channel Dual Polarized electro-magnetic field, with the same antenna (usually called H- and V-signals). Co-Channel Dual Polarization (CCDP) doubles the capacity of a wireless transmission network with a clear advantage to the network operator at the dense frequency bands, both in terms of capacity and in terms of frequency re-use.
Microwave radio energy travels in waves, transmitting in both horizontal and vertical directions. This physical phenomenon enables the transmission of RF waves on both polarizations at the same time. Co-channel dual-polarization operation provides two parallel communication channels over the same link with orthogonal polarizations, thus doubling the link capacity. Separate and independent signals are transmitted over the same wireless channel using a single cross polar antenna.
Despite the orthogonality of the two signals, some interference between the signals almost inevitably occurs, due to imperfect antenna isolation and channel degradation (typically rain depolarization effects).
The main technique to use the CCDP channels, and cancel the effects of the interference is called Cross polarization Interference Cancellation (XPIC).
FIG. 1 shows the high-level block diagram of the receiver side of a known XPIC system.
The system 1 comprises a cross-polarization antenna 2 which receives two orthogonally polarized radio signals, a vertically polarized signal 4a and a horizontally polarized signal 4b, which are both QAM modulated.
A vertical modem 3a and a horizontal modem 3b are provided for respectively receiving the vertically polarized signal 4a and the horizontally polarized signal 4b detected by antenna 2. In particular, the modems contain demodulators 5a and 5b for performing demodulation of a base-band signal and the demodulated base-band signals so obtained are exchanged between the two modems, so as to carry out the cross-polar cancellation. Specifically, after an Analog-to-Digital conversion and band-pass filtering 6a and 6b of both the polar and the cross-polar signals in both modems, the subtraction of the cross-polar signal from the polar signal is performed in nodes 7a and 7b, thus canceling the cross-polar interference. The cross-polar canceller of each modem processes and combines the signals from the two receiving paths to recover the original, independent signals.
A modern trend with existing technology is to use wireless system with Adaptive Modulation functionality. The use of Adaptive Modulation offers an increased capacity over the same radio frequency channel, during periods with favorable propagation conditions. This is obtained using modulation schemes higher than those used to dimension the link for a guarantee capacity and related to a licensed spectrum efficiency class. This results in a higher capacity even with a lower availability due to reduced link budget (according the higher Bit Error Rate threshold and reducing transmission power to improve linearity).
Nowadays the technology challenge is to develop microwave radio links that are able to simultaneously support both functionalities, Adaptive Modulation and XPIC, to assure more and more traffic capability and a higher optimization of the radio link performances.
Problems with the existing solutions are related to the fact that the XPIC system previously described, albeit very efficient, is not fully optimized for Adaptive Modulation function and for new dynamic services.
In fact, using the XPIC solution above, with a fixed modulation radio link, no intercommunication channels between the two modems (managing horizontal and vertical polarization) are needed. The Digital Signal Processing block in the modem, which performs the cross polarized cancellation, works independently from the status of the modem which is processing the cross polarization.
On the contrary, if XPIC were combined with Adaptive Modulation or with these new kind of dynamic services, some communication between the two modems would be required, for sharing between them additional information such as modulation order, traffic rate, and any other available information that must be useful for Adaptive Modulation purposes.
The main disadvantage of the present solutions is hence the absence of a native communication channel between the two modems, unless to install extra wires or cables between the equipment, leading to extra production costs and greater complexity in term of installation.